The present invention relates to a method and an apparatus for deionizing water, such as a condensate in a secondary cooling water system of a pressurized water reactor (PWR) nuclear power plant.
A nuclear power plant may have a pressurized water reactor (PWR). In the PWR, the reactor generates heat, which is transferred into primary coolant. The heat in the primary coolant is further transferred into secondary cooling water through a heat exchanger. The heat in the primary coolant boils the secondary cooling water, thereby rotating a turbine for generating electricity. In other words, the heat exchanger serves as a steam generator for the secondary cooling water. The steam is cooled to a liquid, and the secondary cooling water circulates in the secondary cooling water system. However, the deposition of a scale in the heat exchanger can prevent smooth generation of electricity. There is provided in the secondary cooling water system an apparatus for deionizing a condensate containing an ion exchange resin so as to remove trace ions in the secondary cooling water.
Conventionally, the ion exchange resin used in the apparatus for deionizing the condensate is a gel-type ion exchange resin having a crosslinking degree of 8 to 10% or a porous-type ion exchange resin having an exchange capacity being equivalent to the gel-type ion exchange resin
However, the conventional ion exchange resins have a problem that the ion exchange capacity is limited. Therefore, a regenerant is required to pass through the ion exchange resin to regenerate and wash the ion exchange resin. The procedures make the operation of generating electricity cumbersome, thereby increasing operation costs. Furthermore, during the regeneration of the ion exchange resin by the regenerant, the regenerant may remain in the ion exchange resin, which could elute into the secondary cooling water system during the operation of introducing fresh, secondary cooling water into the system, thereby decreasing the quality thereof. Similarly, organic impurities eluted off from the resin decrease the quality of the condensate.
The whole disclosures of U.S. Pat. Nos. 5,788,828, 5,593,554, 4,814,281 and 4,251,219 are incorporated herein as reference.
In view of the aforementioned problem, one aspect of the present invention provides a method and an apparatus for deionizing a condensate in a secondary cooling water system in a PWR nuclear power plant, which allows the condensate to circulate for a longer period of time and to improve the quality of the condensate.
In accordance with one aspect of the present invention, there is provided a method for deionizing water, comprising the steps of:
providing a nuclear power plant comprising a pressurized water reactor, a primary coolant system operatably connected to the pressurized water reactor, and a secondary cooling water system operatably connected to the primary coolant system; and
passing a condensate in the secondary cooling water system through a mixture of an anion exchange resin and a cation exchange resin having a crosslinking degree of about 12 to 16%.
Preferably, the anion exchange resin and the cation exchange resin form a bed. The cation exchange resin may have a crosslinking degree of about 14%. The cation exchange resin may be a gel type. Preferably, the cation exchange resin comprises a plurality of particles having a substantially uniform particle diameter.
Preferably, the method further comprises the step of passing the condensate through a prefilter whereafter the condensate passed through the prefilter is passed through the mixed bed. The prefilter may comprise at least one of a filter assembly containing hollow-fiber membranes, a filter assembly containing a precoatable filter element and a filter assembly containing a pleated, filter element.
According to the second aspect of the present invention, there is provided an apparatus for deionizing water, comprising:
a secondary cooling water system for a PWR nuclear power plant having a mixture of an anion exchange resin and a cation exchange: resin having a crosslinking degree of about 12 to 16% therein.
Preferably, the anion exchange resin and the cation exchange resin forms a bed. Preferably, the cation exchange resin is a gel type. The cation exchange resin may have a crosslinking degree of about 14%. The cation exchange resin may comprise a plurality of particles having a substantially uniform particle diameter.
Preferably, the secondary cooling water system further comprises a prefilter upstream of the mixed bed. The prefilter may comprise at least one of a filter assembly containing hollow-fiber membranes, a filter assembly containing a precoatable filter element and a filter assembly containing a pleated, filter element.